Human liver disease caused by the hepatitis C virus (HCV) has emerged over the past decade as one of the most difficult challenges facing the worldwide medical community. Elucidation of the viral sequence in 1989 (Choo, et al. Science 244, 359–361 (1989)) initiated the era of concerted study of HCV; presently it is estimated that up to 175,000,000 people are infected (Sarbah, et al. Cell 62, 447–456 (1990)). HCV is the most common type of chronic viral hepatitis with an estimated prevalence of 1–2% in developed countries. Chronic HCV hepatitis leads to liver cirrhosis in at least 25% of affected patients and after development of cirrhosis it is estimated that hepatocellular carcinoma develops in 1–4% of patients each year. In North America HCV is currently the most common indication for liver transplantation.
Currently antiviral therapy with combination interferon and ribavirin is effective in selected patients, but many either fail to respond or tolerate therapy poorly, underscoring a need for improvement. Sustained response rates for interferon monotherapy range from 20–25%, while combination therapy with interferon and ribavirin has shown sustained response rates of up to 40%. Although newer antiviral drugs targeting different parts of the viral genome are under development, progress has been severely hampered by the lack of a robust cost-effective animal model of HCV. The only natural hosts for HCV are humans and chimpanzees, neither of which is suitable for large scale antiviral testing.
The lack of a reproducible small animal model for HCV infection has further limited the investigation of various immune factors contributing to the disease, as well as vaccine candidates for the immunotherapy of chronic HCV infections. In the case of HCV infection, a number of reports have demonstrated the presence of Th1->Th2 switch and HCV antigens specific CD4+ and CD8+T cells in in vitro studies on T cells isolated from the HCV infected individuals. On the other hand, non-viremic HCV infected patients have been found to stimulate strong Th1 response to multiple HCV antigens even many years after infections, suggesting that control of HCV replication may depend on effective Th1 activation (Cramp et al. Gut 44:424–429 (1999)). Resolution of these questions to provide a better understanding of the immune response to HCV, and thus insight as to the development of effective vaccines and therapies, can not be easily reached without a suitable animal model.
Over the past several years, significant advances have been made in the development of animal models for hepatitis B virus. However, despite their similar sounding names, human hepatitis B virus (HBV) and human hepatitis C virus (HCV) are completely different viruses, and thus research regarding HBV infection can not be readily extrapolated to HCV infection. Both viruses are referred to as “hepatitis” viruses primarily because HBV and HCV infect and replicate in the liver. Aside from this, HBV and HCV are no more alike than are HIV and EBV, which each affect the immune system. In fact, HBV and HCV are so different that they are not even member of the same phylogenetic family. HBV is a member of the hepadnavirus family with a genome of double-stranded DNA, whereas HCV is a member of the flavivirus family, which is based on a single positive-stranded RNA genome.
HBV and HCV also differ in their infectivity. HCV is less infectious than an equivalent dose of HBV, as evidenced by the differences in acquisition rates in hospital personnel after needlestick injuries. HBV infections occur in 2–40% of HBV-contaminated needlestick events, while HCV infections occur in only 3–10% of HCV-contaminated needlestick events. These observations suggest that HCV is about three to four times less infectious than HBV (Shapiro Surgical Clin North Amer. 75(6):1047–56 (1995)).
HBV and HCV differ greatly in their requirements for replication as well as in the viral load during infection. HBV is capable of replicating in less differentiated systems (e.g., HepG2 cells, Sells et al. Proc. Natl. Acad. Sci. USA 84:1005 (1987)). In contrast, HCV replication may depend upon the presence of nontransformed hepatocytes (see, e.g., Ito et al. J. Gen. Virol. 77:1043 (1995)). The viral titers of patients infected with HCV are generally lower than those of HBV-infected patients. Patients infected with HBV have levels ranging from 105 to 109 particles per mL, compared to 102 to 107 particles per mL in HCV infections. These differences in viral titer may be due at least in part to the relative clearance rates of viral particles. In addition, the number of viral copies per cell is also very low in HCV infection (e.g., generally less than 20 copies per cell (Dhillon et al. Histopathology 26:297–309 (1995)). This combination of low viral titers and low number of viral copies per cell means that a significant number of human hepatocytes must be infected and producing virus for the infection to even be detected within serum.
The limited host range of human HBV and human HCV has proved problematic in the development of in vitro and in vivo models of infection. Humans and chimpanzees are the only animals susceptible to human HBV infection; human, chimpanzees, and tree shrews are susceptible for infection with human HCV (Xie et al. Virology 244:513–20 (1998), reporting transient infection of tree shrews with HCV). Human HBV will infect isolated human liver cells in culture (see, e.g., Sureau Arch. Virol. 8:3–14 (1993); Lampertico et al. Hepatology 13;422–6 (1991)). HCV has been reported to infect primary cultures of human hepatocytes; however, the cells do not support the production of progeny virions (Fournier et al. J Gen Virol 79(Pt 10):2367–74 (1998)). The development of a satisfactory in vivo model is required in order to provide a more clinically relevant means for assaying candidate therapeutic agents.
The extremely narrow host range of HBV and HCV has made it very difficult to develop animal models. Current animal models of HBV and HCV either do not involve the normal course of infection, require the use of previously infected human liver cells, or both (see, e.g., U.S. Pat. Nos. 5,709,843; 5,652,373; 5,804,160; 5,849,288; 5,858,328; and 5,866,757; describing a chimeric mouse model for HBV infection by transplanting HBV-infected human liver cells under the mouse kidney capsule; WO 99/16307 and Galun et al. J. Infect. Dis. 172:25–30 (1995), describing transplantation of HCV-infected human hepatocytes into liver of immunodeficient mice; Bronowicki et al. Hepatology 28:211–8 (1998), describing intraperitoneal injection of HCV-infected hematopoietic cells into SCID mice; and Lerta et al. Hepatology 28(4Pt2):498A (1998), describing mice transgenic for the HCV genome). Infection by human HBV is fairly well mimicked by infection of woodchucks with woodchuck hepatitis virus (WHV) and by infection of Peking ducks with duck hepatitis virus (DHV). WHV-infected woodchucks and DHV-infected ducks have been successfully used to identify drugs effective against human HBV infection of humans. However, no analogous animal model of infection has been identified for human HCV.
In the absence of a practical non-human host, the most desirable animal model would be a chimeric animal model that allowed for infection of human liver cells through the normal route of infection, preferably a mouse model susceptible to viral infection through intravenous inoculation and that could support chronic infection. Unfortunately, the development of mice having chimeric livers with human hepatocytes susceptible to HBV or HCV infection, and sustaining viral replication and virion production at clinically relevant, sustainable levels has proven no simple matter. The field of xenogeneic liver transplantation has moved very slowly and met with many obstacles.
In order to study neonatal bleeding disorders and hypofibrinogenemia, a mouse transgenic for an albumin-urokinase-type plasminogen activator construct (Alb-uPA) was developed (Heckel et al. Cell 62:447–56 (1990); Sandgren et al. Cell 66:245–56 (1991)). The Alb-uPA transgene includes a murine urokinase gene under the control of the albumin promoter, resulting in the targeting of urokinase production to the liver and producing a profoundly hypofibrinogenemic state. This transgene was also found to be associated with accelerated hepatocyte death. Later work with this transgenic animal demonstrated that individual hepatocytes that spontaneously deleted the transgene acquired a significant survival and replicative advantage, resulting in repopulation of the liver with these nontransgenic cells Sandgren et al., (1991), supra). The Alb-uPA transgenic mouse has proved amenable to transplantation with liver cells from non-transgenic mice (Rhim et al. Science 263:1149–52 (1994)). The Alb-uPA transgenic mouse was also successfully used to produce mice having chimeric livers with rat hepatocytes (Rhim et al. Proc. Natl. Acad. Sci. USA 92:4942–6 (1995)) or woodchuck hepatocytes (Petersen et al. Proc. Natl. Acad. Sci. USA 95:310–5 (1998).
However, these developments were still a long step away from the development of an animal model susceptible to HCV infection. Production of mouse having a xenogeneic transplant from another member of the Rodentia family is not nearly as difficult or unexpected as production of a mouse having a xenogeneic transplant from an animal of a different family, e.g., a human, much less would one expect that a high degree of chimerism could be accomplished, or that such chimeric animals might support HCV infection. For example, hepatocyte growth factor (HGF) is the most potent stimulus of hepatocyte regeneration in vivo; in comparing sequence data, mouse HGF was shown to have 98.5% amino acid sequence homology with rat HGF, and only 90.9% with human HGF (Liu et al. Biochim et Biophys Acta 1216;:299–303 (1993)). There were no guarantees of success.
There is a need in the field for a human-mouse liver chimera susceptible to chronic infection with HCV and with viral production at clinically relevant levels. The present invention addresses this problem.